During January, 1990, five cats from a recently imported colony of 48 cats at the NIHAC developed fatal respiratory tract infections. Bacterial cultures of lung tissues yielded CDC Group EF-4 from four cats. Although sporadic cases of feline pneumonia associated with this poorly defined bacterial group have been reported, this outbreak within a single colony is unique. This study served: (1) to further characterize, by biochemical reactions and SDS PAGE analysis, the case-associated bacterial isolates by comparing them with control EF-4 strains obtained from other sources; (2) to assess the pathology of the lesions associated with EF-4 infection; and (3) to review the medical histories and clinicopathologic findings from cats which died of EF-4 pneumonia and from cats which died from other causes to determine if there were any underlying diseases which contributed to the pneumonia outbreak in this colony. SDS PAGE analysis showed that NIH cat EF-4 bacteria isolates share an identical protein profile, which is unique from profiles of other control EF-4 strains. Necropsy findings include bronchopneumonia (necrosuppurative to pyogranulomatous) and pleuritis (fibrinous to fibropurulent) due to EF-4 bacterial infections. Feline leukemia virus infections, along with shipping stress, may have contributed toward this outbreak of EF-4 pneumonia.